1. Field of the Invention
The present invention relates generally to integrated circuit fabrication, and more particularly but not exclusively to methods and apparatus for magnetron sputtering.
2. Description of the Background Art
Physical vapor deposition (PVD) has been widely used in forming films on a wafer surface during fabrication of integrated circuits. PVD involves physical vaporization of atoms from a target surface using bombarding energetic particles that are usually ions of a gaseous material accelerated in an electric field. Planar magnetron sputtering uses a magnetic field to confine the motion of secondary electrons to near the target surface. An example planar magnetron sputtering apparatus 100 is schematically shown in FIG. 1.
In the planar magnetron sputtering apparatus 100, a substrate 112 is supported by a pedestal 110. Substrate 112 may be a semiconductor wafer, while the pedestal 110 may be a chuck that is vertically moveable towards a target 120. The planar target 120 comprises a material to be deposited on the substrate 112. The target 120 may comprise aluminum, titanium, tungsten, or tantalum, for example. A main magnet assembly 122 is placed behind the target 120 to generate the main magnetic fields for sputtering. The main magnet assembly 122, which has a conventional asymmetric configuration, is rotated by a motor 124 at a rate of about 2 revolutions per second. A magnetic field is superposed on the cathode with the target 120 in a sputtering chamber filled with Ar (argon) gas. Ar ions generated in the glow discharge are accelerated at the cathode and sputter the target 120, resulting in the deposition of thin films on the substrate 112.
A problem with the magnetron sputtering apparatus 100 is that the deposited films tend to shift from their intended location on the patterned substrate. FIG. 2 shows a plot 200 illustrating a rotational shift problem, in which the lines rotating in the counter-clockwise direction represent movement of the deposition flux on a substrate centered at zero (0,0) coordinates. The plot 200 is a vector plot, and the scale of the vectors is not shown for clarity of illustration (note that the shift is in the order of microns). The rotational shift illustrated in FIG. 2 increases with increasing radius and the magnitude of the shift changes throughout the life of the target. This shifting problem affects different features including alignment marks employed in lithography. Unless the rotational shift is prevented, minimized, or counteracted, alignment marks will shift on the substrate with each deposition step. Although alignment mark shifts may be compensated by adjustments in the lithography process, the problem becomes more difficult to deal with as feature sizes get smaller and the deposited films get thicker.